System and method for photolithography in semiconductor manufacturing

ABSTRACT

A method for producing a pattern on a substrate includes providing at least one exposure of the pattern onto a layer of the substrate by a higher-precision lithography mechanism and providing at least one exposure of the pattern onto a layer of the substrate by a lower-precision lithography mechanism. The exposures can be done in either order, and additional exposures can be included. The higher-precision lithography mechanism can be immersion lithography and the lower-precision lithography mechanism can be dry lithography.

BACKGROUND

The present disclosure relates generally to the manufacturing ofsemiconductor devices, and more particularly to a photolithographyprocess in semiconductor manufacturing.

Since the inception of the semiconductor industry, photolithography hasbeen used for forming the components of integrated circuits. Generally,light beams pass through a mask, which has been patterned with amagnified image of the relevant integrated circuits. The light beams arethen focused by a projection lens on to a wafer, resulting in an imageof the integrated circuits in the photoresist layer of the wafer.

Among other factors, the resolution of the image is related to theradiation wavelength and the numerical aperture of the optical system.Specifically, it is desirable to achieve a combination of a smallwavelength and a large numerical aperture for printing dense circuits.

Enhancements are often needed to accommodate the increased density ofintegrated circuits. Some enhanced lithography techniques focused onreducing the radiation wavelength. Currently, state of the artlithography systems use 193 nm as the radiation wavelength for producingsemiconductor devices that include more than one half billiontransistors on each device.

However, it is impractical to continue reducing the radiationwavelength, as light beams with a wavelength smaller than 193 nm areabsorbed by, rather than pass through, projection lenses that convey thelight beams onto the wafer.

Therefore, to continue the advancement of semiconductor fabrication, itis desirable to further enhance the lithography by, for example,improving the numerical aperture of the optical system. One suchenhanced lithography technique that achieves an improved numericalaperture of the optical system is immersion lithography. In immersionlithography (also known as wet lithography), water is inserted betweenthe projection lens and the wafer (in contrast, air is permeated betweenthe projection lens and the wafer in dry lithography). Since water has arefractive index of 1.4, the resulting numerical aperture of the opticalsystem is increased by a factor of 1.4. Accordingly, image resolutionmay be significantly enhanced.

Although immersion lithography works well in increasing image resolutionduring semiconductor fabrication, multiple exposures are still oftennecessary for the required critical dimension (CD) uniformity and endcapperformance with respect to certain semiconductor devices. The costsassociated with enhanced lithography techniques like immersionlithography can therefore be undesirable, especially with multipleexposures.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 illustrates a method of photolithography for implementing one ormore embodiments of the present invention.

FIG. 2 illustrates a photolithography system for implementing one ormore embodiments of the present invention.

FIGS. 3-6 illustrate selected components of a photolithography systemfor implementing one or more embodiments of the present invention.

FIGS. 7 a-7 b illustrate photoresist patterns for implementing one ormore embodiments of the present invention.

FIGS. 8 a-8 b illustrate photoresist patterns for implementing one ormore embodiments of the present invention.

FIG. 9 illustrates a wafer for implementing one or more embodiments ofthe present invention.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

In one embodiment, the present disclosure provides multiple exposures toa wafer. One or more of the multiple exposures are provide by ahigh-precision lithographic method, such as immersion lithography, whileone or more other exposures are provided by low-precision lithographicmethods, such as dry lithography. The sequence of the exposures can bedifferent in different embodiments. For example, a high precisionexposure can be done first, followed by a low-precision exposure. Inanother example, the order can be reversed. In a third example, all theexposures can be immersion, with some exposures having a higherprecision than others. Likewise, in another embodiment, all theexposures can be dry, with some exposures having a higher precision thanothers. In such a manner, the cost and precision of a double exposureprocess is balanced to include the precision benefit (e.g., from using193 nm wavelength immersion lithography) of the high-precision exposureand the cost benefit (e.g., from using 193 nm wavelength drylithography) of the low-precision exposure.

Referring now to FIG. 1, shown therein is a simplified photolithographymethod 10 for implementing one or more embodiments of the presentinvention. Step 14 provides a wafer. Pursuant to step 16, at least oneexposure is provided to the wafer by immersion lithography, and one ormore exposures are provided to the wafer by dry lithography pursuant tostep 18.

The method 10 may be utilized in the fabrication of a variety ofsemiconductor devices, such as memory devices (including but not limitedto a static random access memory (SRAM)), logic devices (including butnot limited to a metal-oxide semiconductor field-effect transistor(MOSFET)), and/or other devices. The method 10 may be especiallyapplicable to fabricating semiconductor devices that are smaller than 56nm in feature size. The method 10 will be further described inconnections with FIGS. 2-8.

FIGS. 2-6, which illustrate an exemplary trim process, will now bedescribed to illustrate a double exposure process utilizing amore-enhanced technique such as immersion lithography for a firstexposure, and a less-enhanced technique such as dry lithography for asecond exposure.

Referring specifically to FIG. 2, a simplified exemplaryphotolithography system 20 includes a light source 21 for emitting lightbeams 23 condensed by a condenser 22. A mask 24, which includespatterns, is illuminated uniformly by light beams 27. After passingthrough the mask 24, light beams 25 are focused by a projection lens 26prior to being projected onto a wafer 28.

The first exposure of the double exposure process will now be furtherdescribed. Referring now to FIG. 3, shown therein are selectedcomponents from the system of FIG. 2, which include the wafer 28, whichis provided pursuant to step 14 of the method 10 (FIG. 1); theprojection lens 26; and the mask 24.

In this embodiment, the wafer 28 includes a photoresist layer 302. Thephotoresist layer 302 may be deposited over the wafer by spin-on coatingand/or other processes. In the present example, a photoresist solutionis dispensed onto the surface of the wafer 28, which is spun rapidlyuntil the photoresist solution is almost dry. In one example, thephotoresist layer 302 may be a chemically amplified resist that employsacid catalysis. In this example, the photoresist layer may be formulatedby dissolving an acid sensitive polymer in a casting solution.

The projection lens 26 may include fused silica (amorphous silicondioxide) an/or any other suitable material known in the art.

The mask 24 may be a high precision plate containing microscopic imagesof electronic circuits. The mask 24 may include a variety of materials,such as quartz, soda lime, white crown, and/or other materials.Generally, a layer of chrome may be included on one side of the mask 24,and electronic circuits (frequently referred to as geometry) may beetched in the chrome layer (designated 24 a). In one example, thethickness of the mask 24 may be approximately between about 60 mm toabout 250 mm.

In one example, a liquid (not shown) may be interposed between theprojection lens 26 and the wafer 28 to cover at least a portion thereof.The liquid may include water, doped water (Cr ion), a fluid having a PHvalue that is greater than 7, a fluid having a refractive index that isgreater than 1, and/or other substance. In one embodiment, a liquid film308, which may cover the exposed area of the wafer 28, may be injectedby an opening in a housing for the projection lens 26.

Pursuant to step 16 of the method 10, the first exposure of the wafer 28is provided by immersion photolithography. In one example, the immersionlithography may include a light source (not shown), such as anelectromagnetic source with a wavelength that is not greater than 250nm, and/or other light sources. In one embodiment, the wavelength of thelight beams emitted from the light source is about 193 nm, 157 nm,and/or other figures. It is contemplated that the numerical aperture(NA) of the optical system may be larger than about 0.75.

Referring now to FIG. 4, following the first exposure, the liquid film308 may be removed by a vacuum (not shown) and/or other suitablemethods. Thereafter, the photoresist layer 302 may be developed bymethods known in the art, resulting in photoresist structures 304 a, 304b, and 304 c.

Pursuant to step 18 of the method 10 (which may be performed before step16 in some embodiments), the second exposure of the wafer 28 is providedby dry lithography. Referring now to FIG. 5, shown therein are selectedcomponents of a lithography system that includes a mask 402 and thewafer 28′. Again, the mask 402 may be a high precision plate containingmicroscopic images of electronic circuits, and may include a variety ofmaterials, such as quartz, soda lime, white crown, and/or othermaterials. Generally, a layer of chrome may be included on one side ofthe mask 402, and electronic circuits (frequently referred to asgeometry) may be etched in the chrome layer (designated 402 a).

The second exposure may be accomplished by dry photolithography, whichmay include any method known in the art. The radiations wavelengths ofthe dry lithography may be about 193 nm, 248 nm, and/or other figures.It is noted that in some embodiments, the mask 402 may be eliminated toaccomplish a maskless exposure.

In another example, both the first exposure and exposures are allaccomplished by immersion lithography with different numericalapertures. For instance, when exposing a high duty ratio region and alow duty ratio region in a single chip, a first immersion lithographywith larger numerical aperture can be used to expose the high duty ratioregion and a second immersion lithography with a smaller numericalaperture to expose the low duty ratio region. Although the manufacturingcost is more, using the larger numerical aperture to expose the highduty ratio region can provide a better image resolution. On the otherhand, using the smaller numerical aperture to expose the low duty ratioregion can reduce manufacturing cost. In some embodiments, both thefirst immersion and second immersion lithography are performed in sameexposure tool, such as stepper or scanner, to maintain overlay quality.In furtherance of the present example, the line/space pitch in the highduty ratio region is equal to or less than about 0.25 um, and theline/space pitch in the low duty ratio region is greater than about 0.25um.

Referring also to FIG. 6, following the second exposure and development,the photoresist layers 304 a and 304 c of FIG. 5 are removed.

It is noted that in the above example, the first exposure may beprovided by dry lithography, and the second exposure may be provided byimmersion lithography instead. It is also noted that with respect to theabove example, additional exposures, which may be accomplished byimmersion lithography and/or dry lithography, are also contemplated.

It is noted that many applications of the method 10 are contemplated bythe present disclosure. In a second example, the method 10 may beapplied to a process entailing a hybrid pattern of active and dummypatterns, such as a hole printing process using phase-shifting masks(Pad Process). The Pad Process is further described in U.S. Pat. No.6,664,011 B2 to Lin et al., which is hereby incorporated by reference inits entirety.

Referring now to FIG. 7 a, shown therein is a top view of a photoresistpattern 700 according to one or more embodiments of the presentdisclosure. In this embodiment, pursuant to step 16 of the method 10, awafer (FIG. 1) is first exposed by 193 nm wavelength immersionlithography with a first mask, and the photoresist pattern 700 is formedfollowing development of a photoresist layer (not shown). In thisillustration, an active pattern 702 is surrounded by dummy patterns 704.The dummy patterns 704 may include dummy bars, virtual bars for lineend, virtual bars for corner rounding, dummy pads, and/or otherpatterns.

Referring now to FIG. 7 b, pursuant to step 18 of the method 10, thedummy patterns 704 may be eliminated through a second exposure utilizing248 nm wavelength dry lithography. It is contemplated that the secondexposure may be accomplished with a second mask. Alternatively, thesecond exposure may be performed without any mask. In one example, thelight source of the dry lithography may include an electromagneticsource, a beam source and/or other suitable sources. It is noted thatthe second exposure may also be accomplished by immersion lithography.

In a third example, the method 10 may be applied to a blind holeprocess. Referring now to FIG. 8 a, shown therein is an exemplary topview of a photoresist pattern 800 that is formed pursuant to step 16 ofthe method 10, which exposes a wafer by 193 nm wavelength immersionlithography. In this illustration, the photoresist resist pattern 800lacks the pattern 804 (shown in FIG. 8 b). Pursuant to step 18 of themethod 10, a second exposure is provided to the wafer by 193 nmwavelength dry lithography. It is understood that one or moreintermediate steps may be performed (such as depositing an additionalphotoresist layer and/or other steps), as is well known in the art. Thedry lithography may include any methods known in the art with or withouta mask, including but not limited to e-beam exposure. Thereafter, thedesired pattern 804 of FIG. 8 b is formed following the developmentprocess. It is noted that the second exposure may also be accomplishedby immersion lithography. For example, the first exposure is 193 nmwavelength immersion lithography and the second exposure is 248 nmwavelength immersion lithography.

Many variations of the above examples are contemplated herein. In oneexample, if the method 10 is applied to a hybrid pattern comprisingactive and dummy patterns, then the critical dimension (CD) of theactive pattern may include a line CD that is not greater than 90 nm, aspace CD that is not greater than 120 nm, and/or a hole CD that is notgreater than 120 nm. In another example, the method 10 may be applied toa multi-exposure process that includes a single photoresist layer or aplurality of photoresist layers. In a third example, the method 10 maybe applied to a multi-exposure process that includes a single etchingprocess following the multiple exposures. In a fourth example, aplurality of etching steps, each of which is applied to the waferfollowing a single exposure process, may be used. In a fifth example,the method 10 may be applied to a variety of processes, such as line endshortening, removing at least one dummy gate, repairing missingpatterns, decreasing the disparity effect between dense and isolatedpatterns, and/or other processes.

Referring now to FIG. 9, for the sake of example, the wafer 28 of FIG. 3is expanded to show a substrate 910, a dielectric layer 914, and abottom anti-reflective coating (BARC) layer 920.

The substrate 910 may include one or more insulator, conductor, and/orsemiconductor layers. For example, the substrate 910 may include anelementary semiconductor, such as crystal silicon, polycrystallinesilicon, amorphous silicon, and/or germanium; a compound semiconductor,such as silicon carbide and/or gallium arsenic; an alloy semiconductor,such as SiGe, GaAsP, AlInAs, AlGaAs, and/or GaInP. Further, thesubstrate 910 may include a bulk semiconductor, such as bulk silicon,and such a bulk semiconductor may include an epi silicon layer. It mayalso or alternatively include a semiconductor-on-insulator substrate,such as a silicon-on-insulator (SOI) substrate, or a thin-filmtransistor (TFT) substrate. The substrate 910 may also or alternativelyinclude a multiple silicon structure or a multilayer compoundsemiconductor structure.

The dielectric layer 914 may be deposited over the surface of thesubstrate 910. The dielectric layer 914 may be formed by CVD, PECVD,ALD, PVD, spin-on coating and/or other processes. The dielectric layer914 may be an inter-metal dielectric (IMD), and may include low-kmaterials, silicon dioxide, polyimide, spin-on-glass (SOG),fluoride-doped silicate glass (FSG), Black Diamond® (a product ofApplied Materials of Santa Clara, Calif.), Xerogel, Aerogel, amorphousfluorinated carbon, and/or other materials.

The BARC layer 920 may be deposited over the dielectric layer 914 by avariety of techniques, including but not limited to spin-on coating,PVD, CVD, and/or other processes.

In one example, the BARC layer 920 may absorb the light thatinadvertently penetrates the bottom of a photoresist layer (not shown).To perform the light absorption, the BARC layer 920 may include amaterial with a high extinction coefficient, and/or considerablethickness. On the other hand, a high coefficient of the BARC layer 920may lead to the high reflectivity of the BARC layer, which counters theeffectiveness of the BARC layer 920. Accordingly, it is contemplatedthat the BARC layer 920 may possess a coefficient value at approximatelybetween about 0.2 to about 0.5, and may possess a thickness of about 200nm. However, it is noted that other ranges of coefficient values andthickness are also contemplated by the present disclosure.

Additionally or alternatively, an index matching approach may be adoptedfor the BARC layer 920. In that case, the BARC layer 920 may include amaterial with a refraction index and thickness that match those of thelight. In operation, once the light strikes the BARC layer 920, aportion of the light is reflected therefrom. Meanwhile, another portionof the light enters the BARC layer 920 and is transformed into a lightwith a shifted phase, which interferes with the first portion of thelight that is reflected from the BARC layer 920, resulting in thereduction of the light reflectivity.

It is contemplated that the BARC layer 920 may employ both the lightabsorption and index matching approaches to achieve the desired results.In some instances, the BARC layer 920 may simply remain over thedielectric layer 914 and serve as a diffusion barrier for the wafer 18,as the removal of the BARC layer 920 may be difficult to accomplish.

Following the deposition of the photoresist layer, the wafer 28 mayundergo soft bake and a double exposure process (described above inconnection with the method 10).

Thereafter, additional steps are adopted for forming a completesemiconductor device. Since those additional steps are known in the art,they will not be further described herein.

Although only a few exemplary embodiments of this disclosure have beendescribed in details above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this disclosure. Also, features illustrated and discussedabove with respect to some embodiments can be combined with featuresillustrated and discussed above with respect to other embodiments.Accordingly, all such modifications are intended to be included withinthe scope of this disclosure.

1. A method for producing a pattern on a substrate layer, the method comprising: providing at least one first exposure onto the layer by a higher-precision lithography mechanism; and providing at least one second exposure onto the layer by a lower-precision lithography mechanism; thereby producing the pattern on the layer.
 2. The method of claim 1 wherein the higher-precision lithography mechanism is performed before the lower-precision lithography mechanism.
 3. The method of claim 1 wherein the higher-precision lithography mechanism is performed after the lower-precision lithography mechanism.
 4. The method of claim 1 wherein the higher-precision lithography mechanism is immersion lithography and the lower-precision lithography mechanism is dry lithography.
 5. A method for photolithography in semiconductor manufacturing, comprising: providing a first photoresist layer for a wafer; providing a first mask for the wafer; performing a first exposure on the wafer by immersion lithography, wherein the first exposure utilizes the first photoresist layer and the first mask; and performing a second exposure on the wafer by dry lithography, wherein the second exposure utilizes the first photoresist layer.
 6. The method of claim 4 wherein the second exposure is performed before the first exposure.
 7. The method of claim 4 wherein a numerical aperture of the immersion lithography is greater than about 0.82
 8. The method of claim 4 wherein a fluid of the immersion lithography comprises a fluid having a PH value that is greater than
 7. 9. The method of claim 4 wherein the immersion lithography comprises utilizing an electromagnetic source having a wavelength that is not greater than about 250 nm.
 10. The method of claim 4 wherein the second exposure utilizes a second mask.
 11. The method of claim 4 wherein the second exposure does not utilize a mask.
 12. The method of claim 4 wherein the second exposure utilizes a beam source.
 13. The method of claim 4 wherein the second exposure also utilizes a second photoresist layer.
 14. The method of claim 4 wherein the second exposure is for an opening printing process using phase-shifting masks.
 15. The method of claim 4 wherein a single etching process is applied to the wafer following both the first and second exposures.
 16. The method of claim 4 wherein first and second etching processes are applied to the wafer following the first and second exposures, respectively.
 17. A method for double exposure photolithography in semiconductor manufacturing, comprising: providing a first photoresist layer for a wafer; providing a first mask for the wafer; performing a first exposure to the wafer by immersion lithography wherein the first exposure utilizes the first photoresist layer and the first mask, and wherein a wavelength of light beams used for the immersion lithography is not greater than 193 nm, and wherein a numerical aperture of the immersion lithography is greater than about 0.75; and performing a second exposure to the wafer by dry lithography, wherein either a second mask or no mask is utilized for the second exposure.
 18. A method for producing a pattern on a substrate layer having first and second duty ratio regions, the method comprising: performing a first immersion lithography with a first numerical aperture on the first duty ratio region; and performing a second immersion lithography with a second numerical aperture on the second duty ratio region; whereby the pattern is produced on the layer by the two lithography performances.
 19. The method of claim 18 wherein a line/space pitch in the first duty ratio region is equal to or less than about 0.25 um.
 20. The method of claim 18 wherein the first immersion lithography and the second immersion lithography are performed in a common exposure tool. 